1. Field of the Invention
This invention relates to a distance detecting device for detecting a distance between an object and the device by projecting a radiation beam coming from the device and by receiving a reflected radiation beam coming from the object as the radiation beam is projected on and reflected by the object.
2. Description of the Prior Art
A variety of distance detecting devices of the above stated kind have been known. Typical examples of such distance detecting devices of prior arts include a device disclosed, for example, by U.S. Pat. No. 3,435,744 or U.S. Pat. No. 3,442,193. The disclosed device is provided with radiation beam projecting means, i.e. a light source such as a lamp, for projecting a radiation beam toward an object whose distance is to be detected and radiation sensing means, such as photocell means, which is disposed away from the projecting means with a given length and is arranged to receive a reflected radiation beam coming from the object as the radiation beam is projected onto and reflected by the object. Under the condition that the radiation beam is projected from the projecting means, the radiation beam receiving axis or direction of the radiation sensing means or both, the radiation beam receiving axis or direction of the sensing means and the radiation beam projecting axis or direction of the projecting means are changed or swept relative to the object by sweep means from a predetermined angular position to a predetermined direction. During the sweeping or changing of said axis or direction in this manner, when the sensing means comes to receive the reflected radiation beam which is reflected by and coming from the object after the radiation beam has been projected from the projecting means, the sweeping operation of the sweep means is stopped. Then, the distance between the object and the device is detected from the position or an angular position of the sweep means or from the amount of its displacement from said predetermined position. This distance detecting device is included in an automatic focusing system of a photographic camera disclosed by U.S. Pat. No. 3,435,744. Another U.S. Pat. No. 3,443,502 discloses also an automatic focusing system for cameras including a distance detecting means in which: Under the condition that a radiation beam is being projected from projecting means in a manner similar to the arrangement of the above stated prior art, sweep means is arranged to change or sweep the radiation beam projecting axis or direction of the projecting means relative to an object whose distance is to be detected into a predetermined direction from a predetermined angular position. Then, in the same manner as in the above stated prior art example, the sweeping operation of the sweep means is stopped, when sensing means comes to receive a reflected radiation beam coming from the object, and thus the distance between the object and the device is found from the position or angular position of the sweep means or from the amount of its displacement from said predetermined position.
Each of the devices of the prior arts is arranged to detect the distance of an object by sweeping the radiation beam projecting axis or direction of the projecting means and/or the radiation beam receiving axis or direction of the sensing means relative to the object whose distance is to be detected and then by finding a point of time at which the sensing means comes to receive the reflected radiation beam coming from the object. It is therefore, most important for improvement in the accuracy of distance detecting operation to accurately detect a point of time at which the sensing means comes to receive the reflected radiation beam coming from the object. Generally, however, the radiation receiving surface area of the sensing means is limited. On the other hand, it is inevitable that the radiation beam projected from the projecting means, though narrowly throttled it may be, is more or less irregularly reflected by the object. Considering these facts, in order to detect the point of time at which the sensing means optimally receive the reflected radiation beam coming from the object during the above stated process of sweeping, there is no conceivable way other than a method of detecting the point of time at which the output of the sensing means reaches its peak level. For a practical application of such a device, therefore, a key to improve the distance detecting accuracy is to accurately detect that the output of the sensing means has reached its peak level. Generally, however, this is very difficult. In one conceivable method, for example, the output of the sensing means is supplied to a known peak hold circuit and, the level of the output of this peak hold circuit and that of the output of the sensing means are compared by a comparison circuit, during the above stated sweeping process, to detect a point of time at which a condition where the output level of the peak hold circuit is equal to that of the sensing means changes to a condition where the output level of the peak hold circuit is higher than that of the sensing means, so that the arrival of the peak level of the output of the sensing means can be detected thereby. In other words, this method is based on the concept that: The output of the sensing means and the output of the peak hold circuit have equal levels until the former reaches its peak level during the above stated sweeping process, and that after the output of the sensing means has reached the peak level, the output level of the sensing means gradually lowers from the peak level while the level of the output of the peak hold circuit continues to retain the peak level of the output of the sensing means. Therefore, when the output of the sensing means passes the peak level thereof, there obtains the relation of "the output level of the sensing means &lt; the output level of the peak hold circuit (=peak level)". Therefore, it is possible to detect a point of time at which the output of the sensing means has reached its peak level by detecting a point of time at which the output level of the sensing means becomes lower than that of the peak hold circuit. However, there is a fatal drawback in this method. The output level of the sensing means might be lowered as a whole due to decrease in the intensity of the reflected radiation beam incident upon the sensing means. In such a case, it becomes hardly possible to accurately detect the peak level. Besides, most of the distance detecting devices of this type are to be used for automatic focusing in a compact photographic camera. Such a compact photographic camera in general has a limited power source capacity. Therefore, the intensity of the radiation beam to be projected from the projecting means is naturally limited. In addition to this problem, objects to be photographed in most cases irregularly reflect radiation beams. The above stated drawback of the method, therefore, is very serious and the use of it for automatic focusing in a photographic camera hardly ensures unvaryingly satisfactory performance.